1. Field of the Invention
The present invention relates to image forming apparatus which form images on an image receiving medium using a plurality of photosensitive drums such as a color copying machine, etc.
2. Description of the Related Art
There is a color copying machine comprising four photosensitive drums arranged parallelly. In this type of copying machine, four photosensitive drums are arranged parallelly and toner images in different colors are formed on the respective photosensitive drums using yellow, magenta, cyanic and black toners. Each of these toner images is transferred and formed on a single sheet of paper.
In the color copying machine using these four photosensitive drums, an image receiving medium placed on a conveyor belt is brought in contact with the four photosensitive drums one by one and respective toner images are transferred from the drums onto the image receiving medium.
Further, when forming an image other than color images, for instance, forming a black image only, no toner image is formed on the yellow, magenta and cyanic drums and a black toner image is formed and transferred onto an image receiving medium. Thus, an image only in black is obtained.
However, a conveyor belt is normally wound around driving rollers comprising rubber rollers and is moved by rotating the driving rollers. The largest reason for using rubber rollers is to prevent the conveyor belt from slipping against the driving rollers by making the coefficient of statical friction of the rubber rollers with the conveyor belt large.
Because, if the conveyor belt slips against the driving rollers, the moving distances of copying papers being conveyed by the conveyor belt changes, causing a color shift on the image receiving medium in the conveying direction. That is, in order to prevent the conveyor belt from slipping against the driving rollers, it is desirable to use soft rubber rollers with hardness of rubber lowered.
However, if a rubber roller is used, accuracy of the outer diameter of the driving roller drops and the more soft a rubber roller is, more worse the accuracy of the outer diameter of the driving roller will become. If accuracy of the outer diameter of the driving roller drops, the peripheral speed of the roller changes, making the conveying speed of the conveyor belt irregular and finally, a color shift is caused on copying papers in the conveying direction.
When a conveyor belt is used for a long time, its surface becomes dirty as toners and paper powder of the image receiving medium attach thereon and therefore, the conveyor belt is cleaned with a belt cleaning device. However, this conveyor belt cleaning device cleans a belt by bringing a rubber blade in contact with the surface of the conveyor belt and a material having a high contact resistance against a rubber blade is used as the conveyor belt. Therefore, when a conveyor belt is rubbed by a rubber blade of a belt cleaning device which is kept in contact with the conveyor belt, electric charge is left. Unless this residual electric charge is neutralized the residual potential of the conveyor belt becomes high and images are not satisfactorily transferred on the image receiving medium. Furthermore, a problem is also caused that ozone is generated if a corona discharger is used to neutralize the residual electric charge.
In this type of image forming apparatus, there was a problem that the conveying speed of a conveyor belt becomes irregular as its peripheral speed changes if the accuracy of the outer diameter of driving rollers drop and as a result, a color shift of images on an image receiving medium may be caused along the conveying direction of the image receiving medium.
Further, as described above, the image receiving medium is conveyed toward four photosensitive drums by a conveyor belt. However, if the conveyor belt is moved while meandering unwillingly, the image receiving medium is also conveyed while meandering correspondingly and there was a problem that the same images in different colors will be shifted as the images in different colors are transferred sequentially on the image receiving medium as a result of the meandering conveyance.
In order to solve this problem, a regulation plate is provided at both ends of the rollers over which a conveyor belt is put as disclosed in the Japanese Utility Model Laid-open Publication (JITSU-KAI-HEI) 4-7543. The conveyor belt is moved while keeping its both ends in contact with these regulation plates to prevent the conveyor belt from meandering.
In this construction, however, if a distance between two regulation plates provided at the rollers is not in accord with the width of a conveyor belt, a problem described below will be caused. That is, there will be a problem that at a place where the distance between two control plates is wide, it is possible for the conveyor belt to meander and at a place where the distance between two control plates is narrow, the conveyor belt may possibly run over one of the regulation plates and as a result, a color shift will be caused on images on the image receiving medium along the direction perpendicular to the conveying direction of the image receiving medium.
Further, in a conventional image forming apparatus, the rollers are rotated by transmitting the turning force of a motor to one of the rollers having parallel shafts over which a conveyor belt is put and a conveying force is provided by moving the conveyor belt in the rotating direction of the rollers. There was a problem that if the moving speed of the conveyor belt becomes irregular, it is not possible to transfer images from four photosensitive drums at a prescribed position and as a result, a color shift is caused on images on the image receiving medium. In view of this problem, construction to use driving rollers directly as the rotary shaft of a motor without using driving transmission gears, etc. which may cause irregular moving speed of a conveyor belt. That is, a driving roller and a motor are in one united body. There are a belt cleaner, photosensitive drums, image transfer rollers, etc. arranged while kept in contact with this conveyor belt along its surface. These arrangements, however, will become loads when driving the conveyor belt. Further, when processing jammed image receiving medium, the conveyor belt is separated from the state in contact with the photosensitive drums and pulled out of the body of the apparatus. Because of this construction, in order to pull out the conveyor belt easily it is necessary to lower the belt to a location where the motor does not come in contact with the photosensitive drums.
On the other hand, in order to drive a conveyor belt while overcoming loads, a motor needs a large torque. Generally, a motor large in size is used to improve its torque. However, because a roller and a motor for driving the conveyor belt are in one united body as described above, if a large motor is used, it becomes necessary to further lower the conveyor belt to prevent the photosensitive drums and the motor from contacting each other when processing jammed image receiving medium. Thus, there comes out a problem that the entire image forming apparatus will become large in size.